1. Field of the Invention
The present invention relates to a novel thin film transistor and a method for its fabrication. More particularly, the invention relates to a novel thin film transistor having a relatively thick source/drain electrode to a channel and a method for fabricating the same.
2. Description of the Prior Art
In a prior art thin film transistor, as shown in FIG. 1, a gate electrode 1 of polycrystalline silicon (hereinafter, referred to as "polysilicon") is formed on a substrate (not shown)., a gate insulating layer 2 is formed on the gate electrode and the substrate, and a polysilicon layer 3 is formed on the gate insulating layer 2. The polysilicon layer 3 is classified into source and drain electrodes 3A and 3B which are formed at both sides of the gate electrode 1, and a channel region 3C which is located between the source and drain electrodes 3A and 3B, as shown in FIG. 1. Particularly, it can be seen from FIG. 1 that, since the source and drain electrodes 3A and 3B and the channel region 3C constitute only one polysilicon layer, each of the electrodes has the same thickness as the channel region.
In the thin film transistor (hereinafter, referred to as "TFT") having the above mentioned structure, the channel region 3C has to be provided as thin as possible, so as to reduce OFF-current of the TFT. However, since each of the source and drain electrodes 3A and 3B comprises the same polysilicon larger or of the channel region 3C, this results in the source and drain electrodes having formed thinner. Thus, there is a problem that ON-current of the TFT is reduced due to increase in resistance of each of the source and drain electrodes.
In addition, when a following process is performed for forming a metal contact on the source and drain electrodes, there is a further problem that a metal layer can not be directly in contact with the source and drain electrodes because each of the source and drain electrodes is composed of a shallow polysilicon layer.
In case the TFT is embodied as a load in an SRAM (Static Random Access Memory), it is necessary to perform an additional process for forming a drain contact. Thus, when such an additional process is performed for forming a drain contact, there is another problem that a gate oxide layer inevitably becomes damaged.